The present invention relates generally to the field of satellite positioning systems, such as the U.S. Global Positioning System, and more particularly to receiving and tracking signals from satellite positioning system satellites.
Most conventional Satellite Positioning Systems (SPS), such as the Global Positioning System (GPS) receivers utilize serial correlators in order to acquire, track and demodulate signals transmitted from the SPS satellites, such as the GPS satellites. Each transmitted GPS signal is a direct sequence spread spectrum signal. The signal available for commercial use is that associated with a standard positioning service and utilizes a direct sequence biphase spreading signal with a 1.023 Mchip per second spread rate placed upon a carrier at 1575.42 MHz. The pseudorandom noise, or pseudonoise (PN) sequence length is 1,023 chips, corresponding to a one millisecond time period. Each satellite transmits a different PN code (sometimes referred to as a Gold code) which allows the signals to be simultaneously transmitted from several satellites and to be simultaneously received by a receiver, with little interference from one another. In addition, data superimposed on each signal is a 50 baud binary phase shift keyed (BPSK) data with bit boundaries aligned with the beginning of a PN frame; 20 PN frames occur over one data bit period which is 20 milliseconds.
An important operation in processing GPS signals is initial synchronization to the pseudorandom sequence that modulates the carrier. This is conventionally done in a serial fashion using the set of correlators, which search for the epoch of the pseudorandom sequence. A typical initial acquisition strategy involves searching the PN code over each of the 1023 symbols in xc2xd chip intervals, which implies a total of 2046 hypotheses. Furthermore, it is often necessary to search over a range of carrier frequency since Doppler and local oscillator errors would otherwise cause the signals to be undetectable. This results in additional frequency hypotheses to test. For high sensitivity applications, each hypothesis can require a dwell time of many milliseconds, and even seconds in some instances. Accordingly, the acquisition process may be very lengthy without the use of many correlators.
Global Positioning System (GPS) receivers receive GPS signals transmitted from orbiting GPS satellites and determine the time-of-arrival of appropriate codes by comparing the time shift between the received signal and an internally generated signal. The signal comparison is performed in a correlation process which entails multiplying and integrating the received and generated signals. A typical prior art serial correlator circuit utilized in common GPS receivers is illustrated in FIG. 1. The correlator 50 receives an input GPS signal 52 and combines, in multiplier 54, the received signal 52 with an internally generated PN code produced by a PN generator 60. A magnitude squaring (or other detection) operation 56 is then performed on an accumulated set of samples of the combined signal. A micro-controller 58 controls the sequencing of PN chips generated by PN generator 60. According to the system of correlator 50, the received signal 52 is compared to a long sequence of PN chips, one time offset at a time, thus requiring a very long period of time to search over all offsets corresponding to one PN frame.
An alternative method for acquiring GPS signals is to use matched filtering approaches; see, for example, co-pending U.S. patent application Ser. No. 09/021,854, which was filed Feb. 11, 1998 and is entitled xe2x80x9cFast Acquisition, High Sensitivity GPS Receiverxe2x80x9d by the inventor Norman F. Krasner. A matched filter, matched to a full pseudorandom frame, may be thought of as a set of 2046 correlators if xc2xd chip spacing is employed. If one wishes to search for M parallel GPS signals, then M such matched filters may be employed in parallel. The foregoing patent application entitled xe2x80x9cFast Acquisition, High Sensitivity GPS Receiverxe2x80x9d shows examples of various types of matched filters which may be used to implement GPS receivers. While the use of such matched filter GPS receivers is efficient, there is often still a desire to further improve efficiency, especially when certain a priori knowledge of signal parameters is available.
The present invention discloses various methods and apparatuses for acquiring and tracking Global Positioning System signals or other types of satellite positioning system signals with a satellite positioning system receiver which includes a matched filter. In one exemplary method of the invention, a first set of frequency coefficients, which corresponds to a first Doppler frequency of an SPS signal, is determined, and the SPS signal is processed in a matched filter with the first set of frequency coefficients during a first window of time. A second set of frequency coefficients, which corresponds to a second Doppler frequency of the SPS signal, is determined, and the SPS signal is processed in the same matched filter with the second set of frequency coefficients during a second window of time, where the first and the second windows of time occur within a period of time which is not greater than one SPS frame period.
In another exemplary method of the present invention, a first SPS signal is processed in a matched filter with a first set of pseudonoise (PN) coefficients during a first window of time, where the first set of PN coefficients corresponds to the first SPS signal and a second SPS signal is processed in the same matched filter with a second set of PN coefficients (which correspond with the second SPS signal) during a second window of time, wherein the first window of time and the second window of time occur within a period of time not greater than one SPS frame period.
Various apparatuses are also described herein.